View full sizeThis illustration shows what the new dome-headed dinosaur Acrotholus audeti may have looked like. In the foreground is an ancient turtle called Neurankylus that Micheal Ryan, David Evans and their colleagues discovered in the same southern Alberta region as Acrotholus.Copyright illustration by Julian Csotonyi

If the new dinosaur that paleontologists Michael Ryan of the Cleveland Museum of Natural History and David Evans of Toronto's Royal Ontario Museum unveiled Tuesday hadn't been so hard-headed, they might never have found it.

Not that the animal was stubborn. Just dense up top. The crown of the 85 million-year-old animal's skull was as hefty as a Stephen King novel – a whopping two-inch-thick dome of bone.

The massive skull cap was all that remained of the otherwise slight, collie-sized dinosaur. The dome's robustness saved it from whatever ravages befell the rest of the skeleton. A graduate student working with Ryan and Evans unearthed the bone from a hillside in the southern Alberta, Canada, badlands in 2008.

After the scientists compared it with a similarly thick skull fragment in the Toronto museum's collection, they determined that the two fossils represented a previously unknown genus, or grouping, and species from the family of dome-headed dinosaurs called pachycephalosaurs. (The family name, pronounced pack-ee-SEF-ah-lo-sore, is Greek for "thick-headed lizard.")

View full sizeGraduate student Caleb Brown holds the fossilized skull cap of the new dome-headed dinosaur Acrotholus audeti soon after he discovered it in 2008.Photo courtesy of Derek Larson

The researchers christened their dinosaur Acrotholus audeti (ack-row-THO-lus aw-DET-eye), a combination of the Greek phrase "highest dome" and the last name of Roy Audet, the Canadian rancher on whose land they dug up the fossil. At more than 80 million years, it's the oldest dome-headed dinosaur yet found in North America, and possibly the world, the two scientists report in the online journal Nature Communications.

While the discovery of a new individual dinosaur is important, Ryan and Evans say the implications of this find are broader, hinting at a hidden, much greater variety of dinosaur types.

Fossils of small-bodied dinosaurs are relatively scarce, compared with bigger relatives like the lumbering duck-billed hadrosaurs and spiky, truck-sized ceratopsids. One interpretation is that there simply weren't as many of those smaller animals around. But an alternate explanation is that their more fragile bones crumbled or were scattered, precluding discovery. The hardy pachycephalosaur domes are an exception; nearly 500 samples of their chunky skulls have been found in North America and Asia.

Based on their analysis, Evans and Ryan contend that scientists have sharply underestimated the diversity of small, plant-eating dinosaurs like Acrotholus.

"If these other small-bodied dinosaur groups are anywhere near as diverse as we're seeing with pachycephalosaurs, they were a lot more diverse than we ever expected," said Evans, the study's lead author.

View full sizeMichael Ryan, curator of vertebrate paleontology at the Cleveland Museum of Natural HistoryCleveland Museum of Natural History

"There are probably multiple other types of small-bodied dinosaurs that [were] never preserved, or we don't recognize pieces and parts of them that we've got stuck in [museum] collections," Ryan said. "We need to spend more time paying attention to this stuff."

Ryan and Evans "have done a great job in . . . opening the window into dinosaur diversity in the Upper Cretaceous," the final 35 million-year period before the animals went extinct, said University of California Museum of Paleontology assistant director Mark Goodwin, who reviewed the study. "They have tested this idea that small- bodied dinosaurs are under-represented, and they have evidence to support it."

Though today it's a desolate swath of rocky outcrops, scrubby vegetation and dry gulches, the area called Deadhorse Coulee in southern Alberta where Ryan and Evans hunt fossils was vastly different 80 million years ago, when Acrotholus roamed the land.

Back then, in the Late Cretaceous Period, a vast inland sea split the North American continent. Along its slowly receding shoreline were lush river deltas and mangrove swamps, comparable to the Florida Everglades. The dense, overhanging ferns and other greenery would have been a paradise for the plant-loving pachycephalosaurs, whose beak-like jaws could snip fresh shoots and crunch the occasional bug.

View full sizeAcrotholus audeti stood about waist-high to a human. It was about 6 feet long and weighed roughly 90 pounds. This illustration shows the location of the prominent skull cap atop the animal's head.Cleveland Museum of Natural History; Royal Ontario Museum

With only the two skull tops and no body bones for Acrotholus, Ryan and Evans had to make some informed guesses about the dinosaur's appearance. Judging by the few other pachycephalosaur skeletal fossils, it would have walked upright on two muscular hind lings, extending its long tail as a counterbalance. Acrotholus' six-foot length was greater than its stubby height. It weighed less than 90 pounds. Wary of roving tyrannosaurs and other big meat-eating predators, Acrotholus probably would have kept out of the open, feeding on the fringes as modern deer do.

Researchers have disagreed for decades about the reason for pachycephalosaurs' unusually thick, domed heads. Paleontologist Edwin Colbert first proposed in 1955 that the heavy bone allowed the animals to head-butt each other, like male bighorn sheep do when vying for mates.

Various scientific analyses have supported or scoffed at the head-butting hypothesis since then, trading arguments about whether skull anatomy and neck angles would have allowed the dinosaurs to endure violent collisions.

The other plausible explanation is that the pachycephalosaurs' bulbous skulls functioned as a visual cue, enabling the animals to recognize their own kind and perhaps, like birds' elaborate plumage, helping females pick desirable partners. Skin color patterns and soft tissue knobs and flaps lost to time may have enhanced the skulls' signaling effect.

"One of my favorite quotes about dinosaurs is if we actually saw them as living animals, not just skeletons, we would never recognize them," Ryan said. "If you look at the skeleton of a rhino or an elephant . . . you would never imagine those giant ears on an elephant or the big, rippling folds on a rhino. I'm sure the dinosaurs had at least that much variation in their soft tissue."

View full sizeIn the foreground of this photo is the pachycephalosaur skull cap the research team discovered in 2008; behind it is the similar specimen that had been in the Royal Ontario Museum's collection for decades.Cleveland Museum of Natural History; Royal Ontario Museum

"These heavy domes – a [tyrannosaur] could eat one of those and it could be exposed to stomach and intestinal acids and still come out looking like a dome," Ryan said. Likewise, they were sturdy enough to withstand erosion's grinding forces.

Scientists had puzzled over the relative scarcity of small-bodied dinosaur fossils compared to the abundance of bones from T rex, Triceratops and other well-known big animals. Was it due to some hidden quirk (what researchers call a "bias')? It could be the lack of fossil-forming conditions in areas where small dinosaurs lived. Maybe erosion hasn't yet exposed rock layers containing small-dinosaur fossils. Perhaps their less resilient bones were pummeled to dust. Or conceivably there just weren't many small-bodied dinosaurs to begin with.

Now there seems to be an answer. To Ryan and Evans, the profusion of domed skulls from Acrotholus and its pachycephalosaur cousins indicates that there were plenty of small-bodied dinosaurs; it's just that the evidence of most of the other types is missing.

"Something we've all known intuitively for years is that anything under about 100 kilograms (220 pounds) of mass, the bones are just too fragile to survive," Ryan said. "Why do we just have skull caps? The obvious answer is that they are big, thick, heavy elements that are resistant to erosion and are going to survive fossilization easily. I think these hard, erosion-resistant bones are a much better indicator of the actual number of animals."

Further support for their position comes from a comparison of the Acrotholus samples to the other pachycephalosaur skull pieces. Some of the oldest and supposedly most primitive pachycephalosaurs had skulls that were thick but flat, or only partially domed. Scientists had assumed that high, full domes evolved later.

But the fully-domed Acrotholus samples are considerably more ancient than the flat-headed ones, which may actually be misidentified juvenile dinosaurs whose still-growing skulls haven't yet formed domes. If Acrotholus, the oldest-known bone-headed dinosaur, had a fully evolved dome, that indicates to Ryan and Evans that there must be even older, more primitive, as-yet-undiscovered ancestors with flatter skulls – a further sign that there were lots of small-bodied dinosaurs.

"If we add these up, I think we see that these animals probably made up a larger percentage of the population," Ryan said. "There has to have been a lot of evolutionary history that we just don't know because we haven't found the specimens."

Locating more fossils of small-bodied dinosaurs – if they survive – probably will require identifying places where fossil-forming conditions were gentler on small bones. That means identifying calm ancient backwater ponds and lakes rather than fast-flowing, fossil-pulverizing ancient riverbeds. Those subtle clues come from poring over maps and painstaking checks of core-drilling samples.

"Can we find those things?" Ryan pondered. "We tend to be like kids in a candy shop. Whatever's big and sticking out of the ground, we dig up, so of course we dig up big dinosaurs. We need to go out with a different search image and look for the smaller things and find the areas where they potentially may be found."

Evans said he hopes the team's results "will encourage people to look more closely at collection and in the field and try to gather more fossil evidence of these small dinosaurs. But it's absolutely a tough challenge, and the fossil record is a puzzle at the best of times. There are some problems that are going to be very difficult to solve."

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